FIGS. 1A to 1E illustrate the various steps of one mode of fabricating an SOI type substrate, which is known in the art.
As shown in FIGS. 1A and 1B, the prior art method consists of oxidizing a source substrate Sou to form a layer of oxide Oxy at its surface, and then to carry out atomic species implantation to define an active layer Cact. The oxide layer is generally fairly thick, of the order of 150 nanometers (nm).
After bonding to a support substrate Sup (FIG. 1C) and detaching the remainder of source substrate Sou (FIG. 1D), a composite substrate is obtained comprising an oxide layer Oxy interposed between support Sup and an active layer Cact, arranged such that oxide layer Oxy has a contact interface with each of the latter, with respective reference numerals I1 and I2.
Finally, after fabrication, a protective layer may be formed on an upper surface of the composite substrate obtained, a layer intended to protect the upper surface during finishing heat treatments on the composite substrate. FIG. 1E depicts the composite substrate covered with a protective layer Cpr and there is an interface I3 between protective layer Cpr and active layer Cact.
From the prior art, methods are already known which can improve, i.e. reduce, Density of Interface Trap (DIT) values, at certain interfaces of a composite substrate. They are principally heat treatments.
One of them, known as Forming Gas Anneal (FGA), consists of carrying out a repair/recovery heat treatment of interfaces at a low temperature, on the order of 450° C., in an atmosphere containing hydrogen and a neutral gas.
When such a method is carried out at 450° C., however, the improvement to the DIT value provided by the method is exerted solely on interface I3 between protective layer Cpr and active layer Cact. The method does not appear to improve DIT values at interfaces I1 and I2, or if it does it is only very slightly. The FGA treatment method loses effectiveness at each interface that is encountered. Thus, the FGA treatment is relatively ineffective for deep-lying interfaces.
A further possibility consists of carrying out an annealing heat treatment at high temperature, i.e. over 900° C. or even over 1000° C. Such a treatment may improve the DIT value at interface I2, but has almost no effect on deeper lying interface I1.
Further, in many prior applications, a thin buried insulating layer (oxide) is desired, for example less than 50 nm, or even less than 20 nm. In that case, the oxide no longer plays the role of electrical insulator alone, but forms an integral part of electronic components that are to be formed on the surface of the composite substrate.
Furthermore, support Sup, located at the bottom, no longer has only a mechanical function, but also has an electrical function. The support substrate Sup may optionally contain buried structures, for example a ground plane, or it may be a composite substrate. As a result, improvements to the DIT values at the two interfaces I1 and I2, located on either side of the buried insulating layer, are desirable.
It is well known that producing composite substrates comprising a very thin buried insulating layer is difficult. Hence, for example, for an SOI type substrate, it is known that the thinner the oxide layer, the higher the number of defects inside the final substrate. This is principally due to the presence of contaminants, polluting particles and gas, encapsulated during bonding at the interface I1.
To improve that bonding, one solution consists of plasma activation of at least one of the surfaces to be bonded, namely that of support Sup and/or insulator Oxy. A high bonding energy is thus obtained, close to 1 joule/meter2 (J/m2), at interface I1, even after an anneal at 200° C. for just 2 hours. But plasma activation deteriorates the electrical characteristics of interface I1, and in particular its DIT values, fairly significantly. Reference in this regard is made to the article by K Schjölberg-Henriksen et al, “Oxide charges induced by plasma activation for wafer bonding”, Sensors and Actuators A 102 (2002), 99-105, which shows the negative effect of plasma activation on the electrical properties of substrates.
Thus, improvements in such treatments are desired and necessary.